mk: Fix cross prefix for powerpc64
mk: Fix cross prefix for powerpc64

Looks like the way to create these executables is to use the standard
`powerpc-linux-gnu-gcc` compiler but with the `-m64` option.

GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust